Photon detection is widely used in applications as varied as process monitoring, optical communications, spectroscopy, remote sensing, quantum cryptography, and tracking. A simple means for improving the sensitivity of photon detection is desired and would result in an increased range of applications with photon detection systems with improved performance.
There has been much work directed at improving the sensitivity of photon detectors. For example, Chappell Brown in the Jan. 29, 2002 EE Times describes the use of FET's with quantum dots to permit the detection of single photons. This work has been done at the Toshiba Cambridge Research Laboratory by a team under the direction of Andrew Shields. In this approach, a two dimensional electron gas is created in the channel of a field effect transistor. The channel is formed at the surface of the FET's heterojunction (e.g., between GaAs and aluminum gallium arsenide). This electron gas is very sensitive to the field on the transistor gate. Instead of a conventional gate on top of the channel, an array of indium phosphide quantum dots is deposited. When a photon hits a quantum dot, it creates an electron and a hole which in turn perturb the two dimensional electron gas, creating a small change in current that can be detected.
As another example, S. Komiyama, O. Astafiev, V. Antonov, T. Kutsuwa, and H. Hirai, Nature 403, 405 (2000) describe a photon detector that can detect photons in the far infrared part of the spectrum. It also uses a quantum dot that is 7000 Angstroms across. The quantum dot is placed in a strong magnetic field that splits the electron energy levels in the dot into two energy levels. An incoming infrared photon can be absorbed by the dot if its energy is equal to the gap between the energy levels. Once the electron is excited by the photon, it leaps onto the terminal of a single electron transistor where it is detected. A 0.1 mm2 device can count one photon every ten seconds. It must be cooled to 1° K in order to detect far infrared photons.
B. S. Karasik, W. R. McGrath, M. F. Gershenson, and A. V. Sergeev, J. App. Phys. 87, 7586 (2000) describe a detector capable of detecting even longer wavelength photons. A millimeter wavelength photon is detected by changing the state of a one micron sized micro-bridge made of a disordered superconducting film. The photon heats the film, causing it to change from a superconducting to conducting state. The photon is detected by this change in resistivity. Like the infrared detector, this detector also requires extensive cooling (in this case, to 0.1° K).
A somewhat similar detector for visible and infrared photons has been developed by G. N. Gol'tsman, O. Okunev, G. Chulkove, A, Lipatov, A, Semenov, K. Smirnov, B. Voronov, and A, Dzardanov, Ap. Phys. Lett. 79 705 (2001). It also uses a superconducting/conducting transition in a micro-bridge made of an ultra thin NbN film. The device is maintained at 4.2° K.
In addition to these relatively new sensitive detectors, there are many more common detectors based on dynode type photomultipliers, single or multi-channel photomultipliers, and avalanche diodes. Some of these are reviewed in the recent article by Wolfgang Becker and Axel Bergmann, “Detectors for high speed photon counting,” which may be accessed at http://www.becker hicki.de/pdf/Spcdetectl.pdf.
Optical amplifiers based on stimulated emission are well known. Stimulated emission can occur in matter in a variety of forms, both fluid and solid. Optical fiber amplifiers are becoming more important now for optical communications [See, e.g., S. Sudo, ed., Optical Fiber Amplifiers. Boston: Artech House, Inc. (1997)]. Optical amplifiers are well described in the U.S. Patent Office subclass 359/333, 349. In these subclasses, optical amplification is described under the headings: Raman or Brillouin process, free electron, bi-stable, correction of deleterious effects, mode locked, particular active medium (e.g., crystal, plasma, fluid, etc.), particular pumping type (e.g., electrical, optical, nuclear, magnetic, etc.), particular resonator cavity (e.g., scanning, confocal or folded mirrors, etc.), multiple pass, and beam combination or separation.
Photon detection is a relatively mature field with both good analytical and experimental support. However, the state of the art is pushed in efforts to increase the sensitivity of detection. Noise and voltage breakdown problems can arise when detectors are made to operate at their limits. In some cases, extreme cooling is required. In other cases, the sensitive detectors can be quite bulky. Devices based on nanotechnology hold considerable promise, but this technology is still in the relatively early stages of development.
Typical high gain photon detectors have gains on the order of 106-108, with pulse response widths of 1 nanosecond. Each input photon yields an output current pulse of peak amplitude on the order of a milliampere. The table below (taken from Wolfgang Becker and Axel Bergmann, “Detectors for high speed photon counting,” that can be accessed at http://www.becker hickl.dcjvdf/spcdetectl.pdf.) shows some typical values for a standard photomultiplier tube (PMT), a fast photomultiplier tube, and a multi-channel PMT:
TABLE 1PMTGainFWHMISERVSERImax (cont)Standard107  5 ns0.32 mA16 mV100μAFast PMT107 1.5 ns  1 mA50 mV100μAMCP PMT1060.36 ns 0.5 mA25 mV0.1μA
In Table 1, ISER denotes the peak current for a “single electron response”, and VSER denotes the average SER peak voltage when the output is terminated with 50Ω. Imax is the maximum permitted continuous output current of the PMT. The Table shows that quite large gains are possible with typical tubes, and that the response time of the tubes is very small and on the order of nanoseconds.
Avalanche photodiodes can also be made quite sensitive if they are operated close to or even slightly above the breakdown voltage. In these devices, the electron-hole pairs generated by the input photons initiate an avalanche breakdown in the diode. Active or passive quenching circuits are used to restore normal operation after an avalanche occurs. These devices have high quantum efficiency in the visible and near-infrared. PIN diodes can be used to detect X-ray photons. Because of their energy, X-ray photons create so many electron-hole pairs in the diode that the resulting charge pulse can be detected by an ultra-sensitive charge amplifier. These devices, however, only have time resolutions in the microsecond range. They can distinguish between different energy photons by the amount of charge generated by each.
Transit time spread and timing jitter occurs in these high gain detectors. For a PMT, there are three major causes: emission at the photocathode, multiplication in the dynode system, and timing jitter in the subsequent electronics. High efficiency semiconductor-type photo-cathodes (GaAs, GasAsP, and InGaAs) introduce transit time spreads of the order of 100 to 150 psec. This is due to the random velocities and random directions of the emitted electrons. The same type of transit time spread is introduced by the dynodes. The timing jitter in the discriminator at the input of a photon counter can also introduce a timing spread. Transit time spread also occurs in avalanche photodiodes, due to the different depth in which the photons are absorbed. The passive quenching circuit can also introduce a spread if the reverse voltage has not completely recovered from the previous avalanche breakdown.
Noise occurs associated with the random thermal emission from the cathode (with the dark current increasing by a factor of 3 to 10 for each 10° C. increase in temperature). Also, most detectors have an increased probability to produce a dark current pulse in a time interval of hundreds of nanoseconds to some microseconds following the detection of a photon.
Solid state detectors have quite respectable noise figures. The sensitivity is often described in terms of the parameter D*, defined as the reciprocal of the noise equivalent power (NEP) of the detector referred to unit area and a 1 Hz electrical bandwidth:D*=(AΔf)1/2/NEP
where A is the detector area and Δf is the electrical bandwidth. Typical commercially available detectors have D*s in the range 108-1012 watt−1-cm-sec1/2. In general, noise can be decreased by operating at reduced gain.
Optical amplification based on stimulated emission has developed rapidly as a result of the rise of optical communication devices. Stimulated emission is enhanced over spontaneous emission in materials that contain a meta-stable state. Thus, ordinarily the spontaneous lifetime of an excited state is on the order of some tens of nanoseconds. However, in some materials, this spontaneous lifetime can be of the order of microseconds to a few milliseconds; in which case the excited state is called a meta-stable state. In that case, the likelihood of a spontaneous transition occurring rather than the desired stimulated emission is small. For stimulated emission to occur, a population inversion has to be created in the host medium. This can be achieved either with optical pumping or electrically (e.g., collisional excitation due to current flow in a discharge).
For the fiber amplifiers which are finding increasing use in optical communications, typical pump lasers include Er, Pr, Nd, and Tm. Pump band gain coefficients can be in the range of 2-6 dB mW. The theoretical limit on the noise figure (degradation of S/N between input and output for an optical amplifier) is 3 dB. For comparison, amplifier gains of 30-40 dB are easily obtained. The noise figure of an optical fiber amplifier actually decreases for a fixed pump length as the gain is increased. [S. Sudo, ed., Optical Fiber Amplifiers. Boston: Artech House, Inc. (1997)].
Depending on the application, an optical collection system may be used in front of the optical amplifier to either narrow the field of acceptance of the system or broaden it. For example, for optical communications, it may be desirable to have a very narrow acceptance angle. This will serve also to minimize background photon noise. For broad area monitoring, on the other hand, it may be desirable to have a wide acceptance angle.
A large aperture collection lens may be used to increase the gathered intensity. However, a limit on the largeness of the aperture may be set by the acceptance angles in the subsequent optical system. The acceptance angle must be larger than the beam divergence at any point in the system. The divergence angle in the subsequent optics is subject to the conservation of the transverse action Δθ D, where D is the diameter of the optical beam at any point in the optical system and Δθ is the associated angular divergence of the beam at that point. If an optical fiber amplifier is used, for example, the allowable divergence of the beam at the fiber end will be determined by the numerical aperture of the fiber. It is understood that a lens system may be replaced by a mirror system or a combined lens/mirror system.
Another method for noise and offset reduction is the chopping technique. Chopping is a modulation technique which shifts the spectra of low frequency stationary processes at multiples of chopper frequency out of the band of interest. The chopping may be achieved with well-known techniques, e.g., mechanically (e.g., rotating wheels), electromechanically (e.g., electrically controlled mirror orientation), by means of electrically controlled crossed polarizer's, etc. As one application, this type of gating may be used with pulsed light sources to reduce the effective background or to distinguish between different signal components.
If a “photon event” comprises emissions from gunfire and chemical explosions (non-nuclear) there is a problem in the detection of radio frequency spectrum. This type of emission is black-body and increases with frequency. At optical frequencies (visible and infrared) the signals are large and are readily detectable by conventional optical detectors. However, optical frequencies cannot penetrate dense clouds, smoke, or foliage. Lower frequencies, down into the radio and microwave frequency bands, can penetrate these obstructions, enabling detection at light speed and offering the possibility of radio localization. But at radio frequencies, antennas are used to capture energy and the antennas suffer from severe limitations. The wavelength-squared dependence on capture area of an antenna means that power collected diminishes with the square of frequency. If an attempt is made to recapture the frequency dependent losses with gain in the antenna, the radiation pattern becomes very directive, requiring prior knowledge of the source location in order to point the antenna, limiting usefulness as a detector. So lower frequencies provide the required penetration but are problematic to detect because they require antennas. As frequency increases, the black body radiation increases with frequency, but the capture area of the antenna decreases with frequency-squared. A sensitive photon detector that does not have a frequency dependent capture area would circumvent the problem of a frequency dependent antenna.
While many techniques have been employed separately to accomplish photon collection, optical amplification, and photon detection, no single system has incorporated all the necessary elements to achieve a photon detection system that is optimized for sensitivity, size, signal to noise, etc. and yet configured with off the shelf components.
There is, therefore, a need for a sensitive photon detector that combines the best features of optical amplifiers with those of state of the art photon detectors to configure a photon detector system that uses off the shelf components, offers smaller size, and provides better signal to noise than is achievable with a single component photon detector.